Many people do not understand the difference between the two types of gray cards. This video shows and explains how to use the cards and gives some examples of situations where you would use them.

You can use the 18% gray card to measure the exposure of a scene. Point your camera at the card, fill the frame with the gray card and take a meter reading. Use that meter reading for your next pictures and they will be perfectly exposed. This is great for shooting snow scenes and doing copy work.

A digital gray card is used for white balance. You use it to get custom white balance in a scene. Cameras don't know what the colour temperature is in your scene. They base automatic white balance on an average of all the colours in your scene, not on the colour of the light falling on it.

If you take a reference photo with the white balance card in the shot, then you can use that to set a custom white balance in image editing, or you can use the card to generate a custom white balance in-camera.

To use it in editing, take a reference photo with the card in it. Doesn't have to be full screen. If taking a portrait, get your model to hold the card near their face. It should be exposed normally. Then in Photoshop or your favourite image editing software, sample the gray card with the neutral colour picker to set your custom white balance, or gray point. You can use curves, or you can use the camera raw white balance tool for this. Once you get this value (white balance correction) then you can apply this setting to all your other photos taken under this colour temperature of light.

All image editing software has this "neutral picker" feature. So with ONE click you will have perfect neutral white balance. That's a great starting point.

If you like you can modify the white balance from that point on, but most of the time I just leave it where it is, unless I want to warm up or cool down my shots.

If you have any questions, just leave a comment with this and I'll answer it here.